This invention relates generally to radio frequency circuits and more particularly to radio frequency limiters.
As is known in the art, in radio frequency systems there exists a trend towards miniaturization and integration of RF components and circuits by use of monolithic microwave and millimeter wave integrated circuit technologies. Such components, including amplifiers, mixers, switches and the like, are finding uses in RF receiving systems. In a typical receiver, the receiver includes a so-called "RF front end" which generally has an antenna coupled to a low noise amplifier. The amplifier generally is fed to an RF mixer which feeds a detector circuit.
It is also known in the art that RF receivers such as those used in radar systems for example typically operate in environments which pose a multiplicity of electromagnetic hazards. In such an environment high RF input signal levels provided by leakage from the radar's transmitter or from hostile jammers for example, pose a threat to those circuit components of a receiving system which are susceptible to burn out as a result of a high incident power level. For example, the low noise amplifier used in the "front end" of a radar receiver generally includes at least on field effect transistor which is succeptable to damage due to high incident power levels.
One type of transistor commonly used in monolithic microwave integrated circuits in the so-called MESFET (metal semiconductor field effect transistor). The MESFET includes source and drain electrodes which are disposed to make ohmic contact to a semiconductor channel region. Disposed in Schottky barrier contact with the channel region and between said source and drain electrodes is a gate electrode. The gate electrode is used to control the electrical conductivity of the channel region between the source and drain electrodes as is commonly known. For high frequency devices such as millimeter wave devices, the gate electrode has a fairly short "gate length" typically of less than 0.25 microns. Even for lower frequency operations such as at microwave frequencies, the gate length is short, (i.e., less than about 1 micron). Because of these small gate lengths, the MESFET is particularly vulnerable to burnout from high level microwave and millimeter wave input signals fed to the gates of the MESFET. This is particularly true for a circuit at the front end of the receiver where the input power level may not be controllable due to the aforementioned electromagnetic hazards. Therefore, such circuits need to be protected from high input power levels.
One approach used in an RF receiver is a PIN-diode limiter circuit. One type of PIN-diode RF limiter circuit is described in a paper entitled "Dual-Diode Limiter for High-Power/Low-Spike Leakage Applications" by R. J. Tan et al., published in IEEE MTT-S, Vol. II, 1990, Page 757. This article describes a limiter having a transmission line with a first end coupled to an input terminal and a second end coupled to an output terminal and at least one, preferably two or more, PIN diodes mounted in shunt between the transmission line and a reference potential. Performance is enhanced when two diodes are placed with opposite polarities a quarter wavelength apart along the transmission line. The limiter operates in two basic modes. While operating in its normal, non-limiting mode, the limiter has a relatively low insertion loss. While operating in a limiting mode however, the PIN diodes ar placed in a forward biased conductive state and as a result the limiter should present a very high insertion loss characteristic to input power signals fed thereto. The limiter is placed between the input signal source such as the antenna for example, and the circuit which needs protection such as the low noise amplifier.
Several problems exist with the PIN diode limiter approach. In general, a PIN diode is a diode having a doping profile that contains a positive doped region (P) spaced from a negative doped region (N) by an intrinsic region (I). A thin I region and a large junction area are necessary for maximum power handling capability when the PIN diode is in its limiting state. A thin I region is also desired to insure rapid recovery from the limiting state to the non-limiting state. On the other hand, a thin I region results in a high capacitance for a given device area thereby increasing insertion loss in the non-limiting state Therefore, a compromise value for I layer thickness must be found.
The PIN diode is used as an RF limiter by applying power to the diode such that both the NI junction and the PI junction are forward biased. Under such a forward bias condition, the I region appears to be n-type material relative to the P material and the I region appears to be P type material relative to the N material. Therefore, the PI and IN junctions behave like two PN junctions. Under forward bias conditions, carriers are injected onto the I region from both the N and P regions. Carriers remain in the I region until hole-electron pairs are formed in a process referred to as recombination. Charge continues to flow into the I region as long as the PI and IN junctions are forward biased. This condition makes the diode appear to be a low resistance path between its terminals. When used as an RF limiter, the PIN diode is coupled between a signal line and ground. Thus, an applied RF input signal is limited if the power level of said input signal is large enough to forward bias the PI and NI junctions of the PIN diodes. Under these conditions the excess power is shunted to ground since the low resistance path between the terminals of the PIN diode is between the RF transmission line and ground. When the applied input signal is below the excess power level, the PI and NI junctions are no longer forward biased and the charge in the PI and NI junctions is removed primarily by the process of recombination of holes and electrons as mentioned above. This process of recombination, causes the PIN diodes to switch from their conducting state to their non-conducting state. The time which is required to switch the PIN diodes from their conducting state to their non-conducting state (i.e. recovery time as described above) slows as junction temperatures increase. Therefore recovery time of a PIN diode is temperature sensitive. Junction temperatures may increase due to the heating which occurs as power is dissipated in the PIN diodes or by a rise in the ambient temperature of the environment in which the limiter is operating.
A further problem is that PIN diode limiters are not easily fabricated as monolithic microwave integrated circuits particularly being integrated with a field effect transistor (FET) on a common semiconductor substrate.
Accordingly, it would be desirable to provide an RF limiter circuit which has the capability to operate as a high speed, high power limiter, having a low insertion loss in the non-limiting state and which can operate up to and including the millimeter wave frequency band.
Furthermore, it would be desirable to provide these same electrical characteristics in an RF limiter circuit which is compatible with monolithic microwave integrated circuit (MMIC) fabrication processes which would permit such circuits to be fabricated as MMICs and further which would permit such a limiter to be integrated on a common semiconductor substrate with the circuit it is being used to protect.